In homosporous ferns individual gametophytes are generally able to form both antheridia and archegonia. No genetic regulation that determines the sex of the haploid generation has been demonstrated. Growth, temperature, light conditions, environmental characteristics, soil conditions, and, in many cases, antheridia-inducing substances can influence the development of antheridia and archegonia (Voeller, 1964; Miller, 1968; Voeller and Weinberg, 1969). We can therefore describe homosporous ferns as having labile sex expression (Korpelainen, 1998).
The antheridia-inducing substances are called antheridiogens, and are products (hormone-like substances) of the metabolism of prothalli. The term antheridiogen characterizes the function but not the chemical composition. In the literature there is a variety of different terms for antheridiogen, for instance, A-substance (Dopp, 1950, 1959, 1962), antheridogen (Pringle, 1961), pheromone (e.g., Naf et al, 1975; Scott and Hickok, 1987), and hormone (e.g., Naf, 1962; Naf et al., 1975; Raghavan, 1989). Schraudolf (1985) distinguished between the pheromonal (effective on neighboring individuals) and the hormonal (effective within an individual plant) phase of antheridiogens. Here, we will use antherid-iogen, the term that is favored in the literature.
5.2 History of discovery
Dopp (1950) was the first to discover a naturally produced substance that induces antheridia formation in young prothalli. He showed that substrate
Biology and Evolution of Ferns and Lycophytes, ed. Tom A. Ranker and Christopher H. Haufler. Published by Cambridge University Press. © Cambridge University Press 2008.
from maturing prothallial cultures of bracken (Pteridium aquilinum) induced antheridia formation in young prothalli of its own species and those of Dryopteris filix-mas. The same was true also when using aquatic extractions. Dopp (1950) interpreted his observation by proposing the presence of a substance he called A-substance, which was active at very low concentrations and was water soluble. It was highly chemically and biologically stable. This pioneering work was supplemented by two later publications (Dopp, 1959, 1962). In 1959 Dopp tested Cryptogramma crispa, Matteuccia struthiopteris, Gymnocarpium robertianum, Pel-laea viridis, Notholaena sinuata, N. distans, and N. vellea. Prothalli from all of these species reacted to the "A-substance" (antheridiogen) of Pteridium aquilinum by forming antheridia. Dopp (1959) observed that if a prothallus of P. aquilinum becomes older and develops a multilayered central part (i.e., becomes meristic), it produces antheridiogen but no longer reacts to it. However, in regeneration experiments, excising parts of female bracken gametophytes or applying incisions, Dopp (1959) realized that only the multilayered, meristic part did not react to the antheridiogen whereas severed parts became male. In 1956, Naf detected another antheridiogen that showed activity in the genus Anemia but not in the species reacting to the antheridiogen of Pteridium (Table 5.1). Schedlbauer and Klekowski (1972) found a third type of antheridiogen in Ceratopteris. The antheridiogen active in Vittaria has not yet been associated with one of the known main types of antheridiogen (Emigh and Farrar, 1977), nor has an additional, possibly different, type of antheridiogen active in Asplenium (Schneller and Hess, 1995).
Since Dopp's initial discovery, the phenomenon of sex determination by antheridiogens has been studied by many different authors and under different experimental conditions (Pour et al., 1998; see also reviews by Naf, 1979, and Yamane, 1998). Based on these investigations, the following general effects of antheridiogen can be described. Prothalli of multispore cultures will remain sterile until some have started to develop a meristic stage (more than one cell layer in the central part of the prothallus). They then start to produce antherid-iogen, which diffuses into the environment. Prothalli that have not yet reached the meristic stage will react to the antheridiogen by producing antheridia and by having a slower rate of growth. Meristic prothalli in many fern species, however, are no longer sensitive to antheridiogen; this is true, for example, in Pteridium aquilinum, Athyrium filix-femina, Sadleria cyatheoides, Bommeria spp., and many other species (Table 5.1). In other taxa such as Asplenium ruta-muraria, A. trichomanes,
Table 5.1 The occurrence of antheridiogens in different fern genera; species are only mentioned when differences of response were observed
Genus or species
Apt antheridiogen Apt antheridiogen Reference
Adiantum Aglaomorpha Anogramma Alsophila Aspidotis densa Asplenium (+ Ceterach)
Dennstaedtia bipinnata Dennstaedtia punctilobula Doodia Drynaria
Voeller, 1964 Näf, 1969 Baroutsis, 1976 Näf, 1960 Greer, 1991
Döpp, 1959; Schneller and
Hess, 1995 Näf, 1956; Schneller, 1979 Voeller, 1964 Näf, 1956 Voeller, 1964
Haufler and Welling, 1994 Chiou and Farrar, 1997 Voeller, 1964
Quintanilla et al., 2005 Chiou et al., 2000 Khare et al., 2006 Voeller, 1964 Voeller, 1964 Haufler and Ranker, 1985 Voeller, 1964 Voeller, 1964 Näf, 1959
Voeller, 1964 Voeller, 1964
Döpp, 1950; Schneller, 1988
Chiou et al., 1998
Chiou and Farrar, 1997
Chiou and Farrar, 1997
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